Various viscous fluid dispensers have been developed for the precise placement of viscous fluid such as a hot melt adhesive. Generally, viscous fluid dispensers have a valve stem with a valve body on its distal end which is disposed on an upstream side of a valve seat and moved in an upstream direction to open the valve and in a downstream direction to close the valve. For purposes of this document, the term "upstream" refers to a direction or location that is toward or closer to the source or fluid inlet and away or further from the fluid outlet of the dispenser; and "downstream" refers to a direction or location that is toward or closer to a fluid outlet and away or further from a source or fluid inlet of the dispenser. Many viscous fluid dispensing applications require that the viscous fluid be applied to the substrate with sharply defined boundaries, that is, the leading and trailing edges of the applied pattern of fluid on the substrate fluid be sharply defined or delimited. Thus, it is necessary that the motion of the valve body be very fast, and the flow of viscous fluid be abruptly started and stopped. With the valve construction described above, when the valve opens, the valve body is moving in an upstream direction against the direction of flow of the fluid and has a tendency to delay and disrupt the flow of fluid out of the dispensing nozzle. Similarly, when the valve closes, the valve body is moving in the downstream direction with the direction of fluid flow and has a tendency to cause a small additional quantity of fluid to be dispensed.
To provide a sharper initiation and cut-off of fluid flow, a "snuff back" valve construction is known. With this construction, the valve body is disposed on a downstream side of a valve seat and moved in the downstream direction away from the valve seat to open the valve and in the upstream direction toward the valve seat to close the valve. Consequently, as the valve opens, the valve body is moving in the same downstream direction as the viscous fluid; and the viscous fluid begins to be dispensed simultaneously with the opening of the valve body. When the valve closes, the valve body is moving in the upstream direction and is effective to sharply cut-off the flow of viscous fluid. While such valves are operated by electric-pneumatic solenoids, due to the relatively short useful life of pneumatic solenoids and their inability to be precisely and repeatably controlled over the long term, it is desirable to provide an electrically operated viscous fluid dispenser of the above-described "snuff-back" design.
Such "snuff-back" electric fluid dispensers are known and are generally of the structure illustrated in FIG. 4. An electrically operated viscous fluid dispenser or dispensing gun 20 comprises one or more dispensing modules or valves 22 mounted on a fluid distribution manifold plate 24 in a known manner. The dispensing valve 22 includes a dispenser body 26 and a fluid dispensing nozzle body 28 having a nozzle 30 through which droplets 32 of the viscous fluid are dispensed onto a substrate 34. Relative motion between the substrate 34 and dispenser 20 is provided in a known manner.
A valve stem 36 is mounted within the dispenser body 26 and has a valve body 38 on its lower, distal end below or downstream of a valve seat 40. The valve body 38 sealingly engages with a valve seat 40 to inhibit the flow of fluid from the dispenser 20. A fluid inlet passageway 42 intersects the interior portion 44 of the dispenser body 26 and is connected to a fluid passage 46 in the manifold 24 which, in turn, is fluidly connected to a pressurized source of viscous fluid 48, such as a hot melt adhesive. Arrows 49 indicate the flow path of the fluid entering through the fluid inlet passageway 46 and through the interior portion 44.
An armature 50 is disposed within the interior portion 44 and is coaxially aligned with, and is often formed integrally with, a proximal end of the valve stem 36. An electromagnetic coil 52 is disposed about the armature 50. A return spring 54 biases the valve stem 36 and valve body 38 in an upstream, upward direction to a closed position at which the valve body 38 sealingly contacts the valve seat 40, thereby interrupting the flow of viscous fluid through the nozzle 30. The return spring 54 is normally a compression spring which is placed under compression within the bore 60 through engagement with an electromagnetic pole 56. When supplied with electrical current, the coil 52 generates an electromagnetic field. The electromagnetic coil 52 must generate an electromagnetic field between the armature 50 and the pole 56 of sufficient strength so as to attract the armature 52 and the pole 56 together. Since the pole 56 cannot move, the armature 52 moves downward against the force of the spring 54, thereby moving the valve body 38 downstream away from the valve seat 40 to its open position.
The design of the dispensing valve 22 of FIG. 4 is known as a normally-closed design for the reason that when the coil 52 is de-energized, the spring 54 maintains the valve body 38 sealingly against the valve seat 40, thereby holding the valve 22 in the closed position. Thus, the valve 22 is normally in, or defaults to, a closed state or position. Therefore, in the event of any electrical power failure to the coil 52, the valve 22 is always mechanically biased to the illustrated closed position. Thus, the valve 22 always defaults to the closed position.
However, to provide that desired normally-closed capability, the armature 50 must be located within the interior 44 above the pole 56; and further, the valve stem 36 must extend through a bore 58 within the pole 56. Those structural features introduce several disadvantages in the operation of the valve. First, the bore 58 in the pole 56 reduces the mass of the pole 56, thereby reducing the effectiveness and strength of the electromagnetic field produced by the coil 52 and pole 56, thereby reducing their capability to move the armature 50. Further, the viscous fluid presents a greater resistance to motion of the portion of the valve stem 36 located within the bore 58 than the portion of the valve stem 36 located outside the bore 58. In addition, the valve stem 36 is substantially elongated to be able to pass through the length of the pole 56, thereby increasing the mass that must be moved by the electromagnetic field. Therefore, the viscous fluid dispenser 20 of FIG. 4 requires that the coil 52 and pole 56 provide a greater electromagnetic force in order to move the armature 50, valve stem 36 and valve body 38 between the open and closed positions.
Second, the interior portion 44 extends over the entire length of the valve body 38, and there are wetted surfaces throughout the entire length of the interior portion 44 of the valve body 38. That large area of wetted surfaces increases the probability of the accumulation of char over the operating life of the dispenser 30. Char is a fluid residue that accumulates on wetted surfaces and is most generally caused by a long term degradation of the viscous fluid. To minimize the potential accumulation of char, the design of the valve of FIG. 2 has a further disadvantage of providing the viscous fluid inlet at the upper end of the dispenser body 26, thereby requiring the viscous fluid to pass through the entire length of the dispenser body 26 prior to being dispensed by the nozzle 30. That long and tortuous fluid flow path not only adds resistance to motion of the armature 50 and valve stem 36 in the viscous fluid, but in addition, the flow rate of the viscous fluid through the dispenser 20 may also be adversely impacted.
In the operation of an electric viscous fluid dispensing gun, the coupling between the coil and the armature is not efficient; and therefore, the electric coil of an electric dispensing valve normally is not capable of providing the same forces as a pneumatic solenoid. That fact in combination with the above described structural features in which the coil 52 and the pole 56 provide a weaker electromagnetic field and the armature 50, valve stem 36 and valve body 38 experience a greater resistance to motion, makes the valve construction of FIG. 4 unacceptable for many viscous fluid dispensing applications.
Therefore, there is a need to provide a more efficient and higher performance design for an electric dispensing gun of the "snuff-back" valve body design as described above.